Method of manufacturing feed pellets and plant for use in the implementation of the method

ABSTRACT

A method of manufacturing feed pellets, and a plant for the implementation of this method have been explained. The aim has been to improve the manufacturing of porous pellets, first and foremost to achieve a better control of the porosity than by known technique. The pellets come from a pelletizing machine ( 1 ) into a pellet chamber ( 3 ) which is kept at a pressure lower than the ambient pressure. From the chamber ( 3 ) the pellets are passed through an outlet ( 5 ) having a gate lock body ( 6 ).

This invention relates to a method of manufacturing feed pellets,whereby moist feed pellets are subjected to negative pressure followedby-a drying process, in order to achieve a more porous pellet and alower temperature load.

The invention also relates to a plant for use in the implementation ofthe method, the plant generally comprising a pellet chamber, preferablyinterconnected downstream of a pelletizing machine, an extruding devicefor pellets or a similar pellet forming device.

Feed pellets for fish and animals are manufactured on an industrialscale in a multistage process. The components are mixed to a dough-likebody, which is formed into pellets by is high pressure and hightemperature, for example in a so-called extruder, after which thepellets are dried and cooled. As warm pellets, typically holding 100 to140° C., are pressure relieved to ambient pressure, the pellets expandbecause of the internal pressure and liquid boiling out of the pellets.The expansion results in the pellets having a porous structure.

The pellets are cut into pieces of desired length. Considerableremaining moisture in the expanded pellet involves that the pellets haveto be dried to obtain keeping quality. Such drying may be done inseveral ways, and some of them should be well known to a person skilledin the art.

The porosity or specific gravity of the completely formed product may bean important criterion of quality of several types of food and feedproducts, including feed pellets for reared fish. The porosity of theproduct is of importance to the possibility of adding liquid nutrientswhich are absorbed into the product; the porosity is further ofimportance to floating capacities in a suitable medium, and it is ofimportance to the texture criterions like crispness, mouth sensation andtoughness. To pellets of fish feed the porosity is important withrespect to the ability of the pellets to absorb oil in the productionprocess, and for the floating capacity/buoyancy in water on feeding.

Existing methods of manufacturing are hard to control accurately, inorder for the product to have the desired porosity or sufficientporosity for the products, feed substances, feed pellets etc. to achievethe desired absorption of fat.

For some products it will be important to be able to control theproduction process towards a minimum of expansion in for examplepellets, whereas the opposite will be the case for other products. Inproducing, among other things, feeds for pets such as for example dogsand cats, and feed for reared fish, this possibility of controlling thedegree of expansion/porosity is essential, because the aim is often toenable addition of as much fat/oil as possible in a subsequentprocessing stage. For fish feed the control of its degree of expansionis particularly important because such feed should, in addition, exhibitdefined sinking capacities in water after its fat/oil absorption.

The most common method of increasing the porosity is to increase themechanical and thermal amount of energy added to the raw materials inthe extruding stage of the manufacturing process. When the initialmixture contains surplus vapour after extrusion, the surplus vapour willexpand and result in greater porosity. It is also possible to supplycompressed gas to the extruder, as disclosed in U.S. patent document No.5,587,193. In patent publications WO 9503711 and 9816121 are mentionedmeans for reducing porosity after the extruding stage by extractingpositive pressure and surplus vapour inside the extruder. In U.S. patentdocument No. 5,527,553 is explained a method, in which the pellets arepassed directly into a warm oil bath at 107-232° C. and cut into adesired length in the oil bath. The degree of expansion of pellets iscontrolled by changing the oil temperature.

An object of the invention is to provide a method and a plant of theinitially mentioned kinds, for use in the manufacturing of porouspellets, whereby a better control of the porosity of the feed productthan by known technique may be maintained.

Another object is to achieve a lower temperature load on the productthrough the processing. Since known methods normally require an extrasupply of energy, such as heat, to achieve increased expansion, theopposite effect of what was normally to be expected has been achieved bymeans of the invention. By the use negative pressure also in thesubsequent drying process and possibly a deep-frying process, anessentially lower temperature load can be achieved for the product thanby conventional methods.

In a method of the kind specified initially, this object is realized byproceeding in accordance with the characterizing part of the followingclaim 1, and by a plant of the initially specified kind for theimplementation of the method, being formed so that it exhibits thefeatures stated in the characterizing part of claim 9.

According to the invention the procedure is such that the pellet isproduced, discharged by or extruded by a pressure which is lower thanthe ambient pressure, pellets being transferred, after a relativelyshort stay by said reduced pressure, to a drying process.

A plant for the implementation of this method comprises a pellet chamberwhich is interconnected in the plant, downstream of the pelletizingmachine, and the plant excels by said pellet chamber being arranged tobe able to be kept at a lower pressure than the ambient pressure, forexample in the order of 100-800 millibar.

In practice this is normally done by extruding pellets in a manner knownin itself, but with the important difference of the extruder dischargingthe pellets into said pellet chamber which works by reduced pressure.The use of reduced pressure will in this connection provide improvedcooling, i.e. a small temperature load on the feed, increasedevaporation of water binding heat. Pellets subjected to reduced pressurewill also expand more than usual, and increased evaporation of watercontributes to the attainment of a more porous pellet. The expansion maybe adjusted by adjusting the negative pressure. So far, experimentscarried out have shown that the pellets, stay by low pressure may be ofa short duration, in typical cases from a few seconds up to one minute,after which the pellets are passed to a drying process.

Experiments have shown that the pellet temperature drops from about 90to about 50° C. when the pressure (inside the pellet chamber) is reducedfrom 1000 to 200 millibar. At the same time the pellet becomes moreporous after the negative pressure treatment, as the density (lessweight per unit of volume) decreases from about 450 to 280 grams perlitre of pellets. Other experiments have shown that also pressure lowerthan 200 millibar has a favourable effect on the control of the porosityof the feed pellets.

The table below shows the results obtained in a series of experimentswith extruded fish feed by the use of the method and plant according tothe invention. The results show a marked increase in the pellet diameterand a reduction in the bulk density as a measurement of expansion whenthe pressure inside the pellet chamber is reduced from 1000 mbar to 200mbar. The temperature of the product also decreases by droppingpressure, as a consequence of increased evaporation. The experimentreferred to, is only illustrative and not limiting to the scope of theapplication. Evaporation Absolute Pellet Bulk Temperature of waterpressure diameter density of pellets (g/kg of (mbar) (mm) (g/l) (° C.)feed) 1000 8.3 460 91.2 5 800 9 416 80.5 6 600 9.1 368 70.4 11 300 10296 59.8 — 200 10.2 284 52 15

In the experiments mentioned the period of stay by negative pressure inthe pelletizing chamber was 20 seconds. Experiments with continuousdischarging from the pellet chamber (i.e. a stay of less than 5seconds), and a stay of 40 seconds have shown corresponding results forexpansion, as those stated above.

A plant for use in the manufacturing of feed pellets excels, accordingto the invention, by the pellet chamber being arranged to allow itselfto be kept at a lower pressure than the ambient pressure, its outletbeing connected to an oil tank or a drying plant, to which the pellet istransferred, and wherein the oil tank or the drying plant is alsoarranged to be able to maintain a lower pressure than that of thesurroundings.

It has proved convenient to let the subsequent drying process also beimplemented by a pressure which is lower than the ambient pressure. Thisstage of the method is advantageous in that it favours the attainment ofthe object aimed at, but this stage is not critical in theimplementation of the method to achieve a satisfactory result. The sameapplies to the deep-frying process which is implemented by reducedpressure in a tank filled with oil, whereby the deep-frying processconstitutes said subsequent drying treatment. For the rest, the dryingprocess may be carried through in a known manner, for example by dryingin air.

Also, the invention comprises a method whereby the pelletizing iscarried out by a first reduced pressure, whereas the subsequent dryingis implemented at a second reduced pressure.

Said first pressure and said second pressure may be identical ordifferent from each other.

As mentioned, reduced temperature will be favourable to temperaturesensitive components, and increased porosity is favourable to thecapacity of the pellets to absorb oil, whether the oil is added inconnection with the deep-frying, or the oil is added after the pelletshave been dried in another way (for example by drying in warm air).

The outlet of the pellet chamber may have a rotatable gate lock bodyarranged thereto, enabling formed pellets to be drawn continuously or inbatches, while, at the same time, the negative pressure is maintained.

According to the invention pellets are produced in a pelletizing machineand passed from there into said pellet chamber which works at reducedpressure. The degree of negative pressure relative to the atmosphericpressure is adjusted with a view to the desired expansion of pellets.This has turned out to provide an essentially better control of theexpansion and porosity, than measures which have to be taken in a knownmanner before or during pelletizing. The reason is believed to be thatin changing single parameters of the pelletizing process, otherparameters are also influenced, which are very important for a goodresult. This is because the pelletizing process creates physical andchemical structures of the raw materials by means of the same measuresthat control expansion (heat, water and pressure). One should perhapsbelieve that the same effect as by the invention could be achieved byincreasing the pressure by pelletizing and producing pellets into freeair with the same pressure drop as the one achieved by the invention.However, such a pressure increase does not have that effect. There willnormally be operated with pressure variations, in for example theextruding process, way over 1 atmosphere (about 1000 millibar), withoutthis affecting expansion and porosity in a manner worth mentioning. Inthe production of animal feeds the pressure before pelletizing will bebetween 15 and 40 atmospheres, depending on the choice of raw materialsand desired quality of the final product. Pressure is one, but not themost essential process parameter for adjusting the expansion.

As an explanation of the surprising effect obtained by the applicationof the invention, a more rapid boiling out of water and subsequenttemperature drop are considered to be the most important ones. The dropin temperature results in the pellet matrix setting, thereby preventingthe shrinking effect which is otherwise to be expected.

The pressure within the pellet chamber may be in the pressure range from0 millibar to right below atmospheric pressure, and will in typicalcases be between 100 and 800 millibar.

According to the method of the invention, porous pellets are produced ina manner known in itself, but with the novel feature of pellets beingdischarged into a pellet chamber which is kept at a pressure lower thanthe ambient pressure, typically in the range from one hundred to eighthundred millibar.

According to the method of the invention, water is removed from thepellets, and the pores are filled with fat in subsequent processingstages.

According to the invention the outlet of known pelletizing equipment hasa pellet chamber arranged thereto, which is arranged to be able to bekept at a lower pressure than the surroundings, and which is providedwith a gate lock opening so that pellets may be drawn continuously or inbatches from the pellet chamber, while the chamber is kept by a reducedpressure.

In the following the invention will be described in further detail bymeans of an exemplary embodiment, and reference is made to theaccompanying drawing, in which the single figure shows a schematic sideview of a plant for the manufacturing of pellets.

In the figure of the drawing the reference numeral 1 identifies apelletizing machine with an outlet 2 which opens into a pellet chamber3. The pellet chamber 3 has a first vacuum pump 4 arranged thereto,which is arranged to maintain the air pressure inside the pellet chamber3 at a first desired value, lower than the ambient pressure. At itslower end, the pellet chamber 3 is provided with an outlet 5, in whichthere is positioned a gate lock device 6 of a known type, so that thelow pressure of the pellet chamber 3 may be maintained while the pelletis discharged. The gate lock device 6 may with advantage be of arotational type, so that pellets may be fed continuously out of thepellet chamber 3.

The outlet 5 is connected to an inlet 7 in the upper part of an oil tank8 which is partly filled with oil, which is not shown. The oil tank 8has a second vacuum pump 9 arranged thereto, which is arranged tomaintain the air pressure inside the oil tank 8 at a second desiredvalue, which is lower than the ambient pressure and normally also lowerthan said first desired value of the pellet camber 3. Further, the oiltank 8 is provided, in a known manner, with a heating element withthermostatic control, possibly an agitator, which is not shown, in orderto serve for the deep-frying of pellets.

1-22. (canceled)
 23. A method for manufacturing feed pellets having aninitial pore volume and fat content said method comprising: A. extrudingpellets from a feed material within a pellet extruder having a dischargenozzle; B. exposing said extruded pellets to a first pressure lower thanambient pressure immediately subsequent to said extruding step, saidexposure to said first pressure occurring in a pellet chamber downstreamfrom said discharge nozzle, whereby said pellets expand and increasesaid pore volume; and C. subsequently adding oil to said pellets toincrease said fat content for said pellets:
 24. The method of claim 23further comprising a drying step.
 25. The method of claim 24 whereinsaid drying step is conducted prior to adding oil to said pellets. 26.The method of claim 25 wherein said drying step is drying in air. 27-30.(canceled)
 31. The method of claim 23 wherein the step of adding oil tosaid pellets comprises coating said pellets with oil.
 32. The method ofclaim 23 wherein the step of adding oil to said pellets comprises addingsaid pellets to a hot oil bath.
 33. The method of claim 23 wherein thestep of adding said oil to said pellets is conducted in a vacuumchamber, said vacuum chamber comprising a hot oil bath.
 34. The methodof claim 33 wherein said vacuum chamber is at a second pressure which islower than ambient.
 35. The method of claim 34 wherein said secondpressure is different than said first pressure.
 36. The method of claim23 wherein the step of adding said oil to said pellets is conducted in avacuum chamber.
 37. The method of claim 36 wherein said vacuum chamberis at a second pressure which is lower than ambient.